1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing optical information in which a focusing error signal and a tracking error signal are detected by using light reflected from an optical information recording medium. The present invention also relates to an optical pickup device using a dual grating to detect a signal with respect to the light reflected from the optical information recording medium.
2. Description of the Related Art
In a first proposed example of an apparatus for recording and reproducing optical information, the apparatus is applicable when a compact disk, an optical disk or a write-once type, etc. are used as an optical information recording medium. However, this apparatus has no function for detecting a signal recorded onto a magnetooptic disk. Further, in this apparatus, a diffraction angle is changed in accordance with the change in wavelength of light, which is a fundamental disadvantage caused by a grating itself such as a polarizing beam splitter grating, a 1/4 wavelength plate grating, etc.
As a second proposed example, there is an optical information recording-reproducing apparatus which can also correspond to a magnetooptic disk. However, in this case, since a dual grating having gratings formed on the front and rear faces of a substrate is used, a difference in optical path is caused between zeroth and first order lights when light is transmitted through this dual grating. In particular, this difference in optical path is increased as the thickness of the substrate is increased. Therefore, when two light spots are detected on the same plane, the spot of the first order light is greater in size than that of the zeroth order light so that it is necessary to dispose a large-sized light-receiving element. When the thickness of the substrate is decreased, the difference in optical path can be reduced and the size of the spot of the first order light is not increased. However, the generating amount of aberration is reduced with respect to the zeroth order light so that it is very difficult to detect a focusing error signal by an astigmatic method.
Further, in the above-mentioned apparatus in the second example, the number of optical parts disposed in a lighting optical system with respect to the light emitted from a semiconductor laser to the magnetooptic disk is increased so that the entire apparatus cannot be made compact and light.
In an optical pickup device using a dual grating having gratings on the front and rear faces thereof, it is possible to dispose light-receiving elements for signal detection in the same place. Therefore, the pickup device can be made compact and light and can be operated irrespective of the change in wavelength of light. However, since light is transmitted twice through the gratings, light utilization efficiency and diffracted wave efficiency are reduced in comparison with those in a structure in which light is transmitted once through the gratings, thereby causing problems about stability in signal detection.